There are eight planets in our solar system. The names for all of them, except for Earth, are derived from Roman and Greek mythology.
Earth’s origins are Germanic and Old English.
The United States Department of Energy’s Web site states that most likely, the word Earth came from the Middle English word, erthe, which came from Old English eorthe; akin to Old High German erda. This then traces back to the Greek eraze from the Hebrew eres, meaning ground.
There are still questions though about who started the naming process. Today when a new planet is discovered the International Astronomical Union (IAU) is in charge of naming all celestial objects. When an astronomer discovers an object they can submit a name to the IAU for approval.
But the word Earth existed before we truly understood planets. Even today we continue to define the criteria.
The definition of planet just recently changed. The old definition described a planet as wanderers that were known only as moving lights in the sky. In August, the IAU further detailed the definition of planets and all bodies in our solar system. This is when Pluto lost its distinction as a planet.
The process of definition and naming has evolved over time. As technology improves, who knows what planet will be found next.